Trouble adding two normal maps to an object
This may not answer your question and I cant even explain what is going on but maybe this can help you. 

Since I cant explain this answer I will take it down if I get yelled at or downvoted. lol 

This is for using 2 normal maps (or more):





I have never plugged a normal map directly into displacement socket. I don't know if that is causing the problem or not.

I attached the .blend file so you can just append the node tree if you want.